This invention relates to a method for lowering the curing temperature of acetylenic substituted polymers. The method of the invention is particularly, but not exclusively, concerned with the curing of acetylenic polyimides.
Aromatic polyimides are widely used as resins for moulding of plastics articles, as adhesives, and as matrices for composite materials intended for service at elevated temperatures. Such polyimides are generally produced by the condensation of a mixture of one or more diarnines with a stoichiometric amount of one or more tetracarboxylic dianhydrides in a suitable solvent such as dimethyl formamide or N-methylpyrrolidone to form a polyamic acid which on heating can undergo cyclodehydration to form the polyimide (for example, as shown in FIG. 1 of the accompanying drawings). Often the intermediate polyamic acid solution is used for coating the articles which are then heated to form the polyimide in-situ.
Whilst polyimides of high molecular weight are required for the development of adequate mechanical strength, it is often preferred, for ease of production and other reasons, to employ a lower molecular weight, oligomeric imide or amic acid containing substituents which can undergo chain extension and crosslinking reactions during thermal processing to form the cured thermoset resin. Examples of such systems already widely used in industry include the bismaleimides and the nadimide-based PMR resins which undergo cure at temperatures near 250xc2x0 C. Many other systems have been described in the literature, but these have apparently failed to obtain commercial acceptance.
One disadvantage of most of these thermoset polyimides is their failure to withstand oxidative degradation on long-term exposure at temperatures above 200xc2x0 C. This is because the crosslinking moieties have generally inferior thermal stability, compared to the oligoimide units, and therefore can act as weak links in the polymeric structure.
One class of thermoset polyimides which does appear to provide adequate thermal stability are those containing phenylethynyl-substituted aromatic species as the reactive moieties, e.g.: 
Such systems have been described in patents assigned to National Starch (e.g., U.S. Pat. No. 5,138,028) and the United States National Aeronautics and Space Administration (e.g., U.S. Pat. No. 5,567,800). However, the processing of these phenylethynyl resins requires curing at much higher temperatures (350-400xc2x0 C.) than those typically required for the cure of the bismaleimide or nadimide resins.
The chemistry involved in the curing of the phenylethynyl (phenylacetylenic) resins has not been conclusively established, but is believed to involve the initial condensation of two or more ethynyl (acetylenic) groups to form a mixture of linear poly-enes or ene-ynes which can then undergo a variety of inter- and intra-molecular addition and substitution reactions to form the crosslinked, fully cured product. The initiation of these reactions probably involves adventitious free radical intermediates, although the ethynes (acetylenes) as a class are not particularly susceptible to radical-induced polymerization.
It is therefore conceivable that processing temperature for the phenylethynyl resins could be lowered, for example, by the addition of free radical catalysts such as peroxide initiators, or by utilising the known activation of alkynes by transition metal complexes. However, these approaches have some potential difficulties.
To obtain void-free products, it is desirable that the resin should be fully cyclized before onset of the cure reactions, and should be free of solvents, amic acid residues, and other species which could evolve volatile by-products during processing. However, the fully cyclized, solvent-free oligo-imides which could serve as precursors for cured resins having the preferred glass transition (Tg) temperatures in excess of 250xc2x0 C. themselves usually have softening or Tg temperatures above 200xc2x0 C. and the chain extension and crosslinking reactions cannot proceed at practical rates below the softening point or Tg of the resin. Most peroxidic initiators undergo rapid and irreversible decomposition well below these temperatures, and so would be inefficient as cure accelerators for practical phenylethynyl resins. The use of transition metal catalysts, on the other hand, would leave undesirable residues which could also promote thermooxidative degradation of the cured resin during service at elevated temperatures.
We have now discovered that the addition of suitable organic disulfides or polysulfides, or elemental sulfur, to phenylethynyl-substituted oligo-imides reduces the onset of cure temperature by 50xc2x0 C. or more, thus enabling the thermal curing of the resins to occur at temperatures at 300xc2x0 C. or below. We have also found that additional improvements can follow from the structural inclusion of disulfide moieties in oligo-imide chains. These additives or disulfide moieties undergo reversible dissociation at 200xc2x0 C. or above, preferably 200xc2x0 C. to 300xc2x0 C., to form thiyl radicals which can react with the phenylethynyl groups to initiate the cure of the resin.
The use of sulfur, disulfides, and polysulfides as additives in the vulcanization of olefinic elastomers is well known. However, there appears to be no previous reports or claims for their use as curing agents in acetylenic polyimides, or more specifically, the commercially promising phenylethynyl polyimides.
According to the present invention there is provided a method for promoting the curing reactions of an acetylenic oligomer or polymer, which comprises curing the oligomer or polymer in the presence of sulfur or an organic sulfur derivative which is capable of thermally generating thiyl radicals during the curing reaction thereby lowering the temperature of cure of the oligomer or polymer.
The organic sulfur derivative can be selected from disulfides and polysulfides of the formula
Rxe2x80x94Snxe2x80x94Rxe2x80x2
wherein (nxe2x89xa72) and the substituents R and Rxe2x80x2 may be substituted or unsubstituted alkyl, cycloalkyl, aryl, arylalkyl, or heterocyclic moieties, and may be the same or different; or derivatives thereof, such as mono- or di-acyl or aroyl disulfides of the formula: 
imidyl (imidoyl) or thiocarbamyl disulfides of the formula: 
wherein R and Rxe2x80x2 are as defined above; or other sulfur-containing species, including sulfenyl derivatives and elemental sulfur, which can generate thiyl radicals when heated to the processing temperature of the resins, typically in the range of 150-300xc2x0 C.
In another aspect the invention provides an acetylenic oligomer or polymer having at least one ethynyl group, characterised in that it comprises an organic sulfur moiety which is covalently bound to, and forms an integral part of the oligomer or polymer and which is capable of thermally generating thiyl radicals during cure of the oligomer or polymer thereby promoting the cure of the oligomer or polymer.
In a further aspect the invention provides a composition which comprises an acetylenic oligomer or polymer having at least one ethynyl group and sulfur or an organic sulfur derivative having an organic sulfur moiety, characterised in that the sulfur or organic sulfur derivative is capable of thermally generating thiyl radicals during cure of the oligomer or polymer thereby lowering the temperature of cure of the oligomer or polymer.
In yet another aspect there is provided a process for producing an acetylenic polyimide oligomer or polymer containing one or more ethynyl group per molecule and containing an aliphatic or aromatic disulfide moiety which is covalently bound to, and forms an integral part of the oligomer or polymer and which is capable of lowering the temperature of cure of the oligomer or polymer, characterised in that a suitable amount of a bis(amino-substituted)hydrocarbyl disulfide or bis(anhydride-substituted)hydrocarbyl disulfide, or any suitable derivative or precursor thereof, is introduced into the mixture of aromatic diamines, tetracarboxylic dianhydrides, and the phenylethynyl-substituted amine or anhydride normally used for the preparation of the oligomer or polymer.
In this specification xe2x80x9csubstitutedxe2x80x9d group means that a group is substituted with one or more non-deleterious groups selected from: alkyl, alkenyl, aryl, halo, haloalkyl, haloalkenyl, haloaryl, hydroxy, alkoxy, alkenyloxy, aryloxy, haloalkoxy, haloalkenyloxy, haloaryloxy, amino, alkylamino, alkenylamino, alkynylamino, arylamino, acyl, aroyl, alkenylacyl, arylacyl, acylamino, heterocyclyl, heterocyclyoxy, heterocyclylamino, haloheterocyclyl, alkoxycarbonyl, alkylthio, alkylsulphonyl, arylthio, arylsulphonyl, aminosulphonyl, dialkylamino, dialkylsuphonyl.